1. Field of the Invention
The present invention relates to a method for producing xcex1-alumina powder. In particular, the present invention relates to a method for producing xcex1-alumina powder having regulated particle size and particle shape, a narrow particle size distribution and a low halogen content, and xcex1-alumina powder produced by such method.
2. Description of the Related Art
Alpha-alumina powder is widely used as a raw material of an abrasive, a sintered body, and the like, and various production methods thereof are known. But, such conventional methods have the following drawbacks.
For example, a method disclosed in Japanese Patent KOKAI Publication No. 131517/1991 (a flux method) produces only particles of xcex1-alumina powder which have a hexagonal close-packed lattice and a particle shape of a hexagonal plate having a ratio D/H of 5 to 40 in which D is the maximum particle size in a direction parallel to the hexagonal lattice plane and H is a particle size in a direction perpendicular to the hexagonal lattice plane, and this method also has a problem that the shape of the particle is not uniform.
Alpha-alumina powder produced by a method disclosed in GB Patent No. 990,801 and German Patent No. 1,767,511 has a non-uniform particle shape and a wide particle size distribution. Alpha-alumina powder produced by a method disclosed in Japanese Patent Publication No. 8929/1968 contains less impurity but has a wide particle size distribution.
A method disclosed in Japanese Patent Publication No. 22886/1982 makes it possible to control the particle size of xcex1-alumina powder by the addition of corundum as a seed crystal. However, this production method is not effective industrially, since it requires a high pressure and a long time.
A method disclosed in Japanese Patent KOKAI Publication No. 97528/1984 can produce xcex1-alumina powder having an average particle size of 1 to 10 xcexcm and a shape with the D/H ratio of about 1, but xcex1-alumina powder contains remaining boron or fluorine which is used as a mineralizer in the production method and includes many agglomerates which are formed in a sintering step.
As described above, the known methods have a drawback that the produced xcex1-alumina powder has a low packing property and cannot be homogeneously packed since the control of the particle size of the produced xcex1-alumina powder is difficult, the particle shape is nonuniform, xcex1-alumina powder includes many agglomerated particles which are polycrystal, or xcex1-alumina powder has the wide particle size distribution.
An object of the present invention is to provide xcex1-alumina powder which does not have the drawbacks of the conventional production method and a method for producing such xcex1-alumina powder.
As a result of the extensive study, it has been found that, when transition alumina or an alumina compound is calcined in an atmosphere containing at least 0.1% by volume of halogen, a particle size and shape of the produced xcex1-alumina powder are well controlled and xcex1-alumina powder having a narrow particle size distribution is obtained and that subsequent dehalogenation provides xcex1-alumina powder having good properties and a low halogen content.
The produced xcex1-alumina powder is found to be advantageously used as a raw material of a high purity sintered body, an abrasive, a raw material of a porous material such as a ceramic filter, a raw material of a single crystal, and so on, since it has a high packing property, can be homogeneously packed and has a low halogen content.
Accordingly, the present invention provides a method for producing xcex1-alumina powder comprising the steps of calcining at least one starting material selected from transition alumina and alumina compounds in a halogen-containing atmosphere which comprises a hydrogen halide gas, a halogen gas or a mixture of a halogen gas and steam and contains at least 0.1% by volume of at least one halogen-containing gas selected from the group consisting of hydrogen halide gas and halogen gas, and removing halogen from a calcined material.
Further, the present invention provides xcex1-alumina powder comprising particles which have a hexagonal close-packed lattice having a polyhedron shape with at least 8 faces, a D/H ratio of from 0.5 to 3.0 where D is the maximum particle size in a direction parallel to the hexagonal lattice plane and H is a particle size in a direction perpendicular to the hexagonal lattice plane, a D90/D10 ratio of 10 or less where D10 and D90 are particle sizes at 10% and 90% accumulation, respectively from the smallest particle size, and a halogen content of 40 ppm or less.